This invention relates to secure or privacy telecommunication systems and more particularly to apparatus for bypassing such a system to enable a subscriber or user to communicate in a conventional manner.
Systems for communications such as telephone systems have employed scrambled data or speech to provide security and hence, to prevent unauthorized persons from listening in to highly confidential or classified information. Essentially, the function of a scrambler or a secure communication system is to render speech unintelligible by processing voice signals in a predetermined manner. Such techniques are sometimes referred to as scrambling and in order to receive a scrambled speech pattern, both parties must possess the necessary equipment to scramble and unscramble the signal according to a predetermined format.
In early systems of this type, voice frequencies were inverted or shifted and these frequencies were converted back to the original form at the receiving end by an unscrambling circuit. Accordingly, anyone having access to the scrambled speech would not understand or be able to decipher the same. Systems for scrambling and descrambling speech or other data have been widely improved and modern systems employ complicated digital techniques. In such techniques, the analog voice signal is converted to a digital signal by means of an analog to digital converter. The digital signal may be further treated by using pulse code modulation techniques to provide a scrambled signal wherein various bits of the signal are indicative of the original analog signal and are placed in a frame format or in various bit patterns according to a predetermined coding sequence. In this manner, such systems are extremely difficult to interpret and hence, even if one obtained access to the scrambled information, one would not know how to break the code.
In general, such systems are called security systems as compared to a privacy system in that due to the complexity of such systems, an interceptor must be familiar with the principles used in scrambling the voice data and must also use sophisticated equipment to discern how the data is arranged. These systems using digital procedures as above mentioned are extremely secure and are virtually impossible to break without having the exact knowledge of the coding techniques employed.
The use of digital security systems in telecommunications is extremely well known and such systems have been used, for example, in secure telephone systems to enable all subscribers to communicate with each other in a completely secure mode. As one can ascertain, such telephone systems are extremely useful in modern day society in allowing various corporate officials to communicate in a secure mode as well as a host of other uses, including military applications as well. These security techniques, when employed in conjunction with a typical telephone switching system, may employ ordinary telephone lines as furnished by the carrier and such lines may be switched through conventional telephone switching networks. Hence, it is, of course, understood that the transmission paths which are controlled by the telephone carrier may be accessible. In any event, the subscribers can still communicate in a completely secure mode due to the fact that each designated subscriber has a desk set or subset which includes a secure communications system such as a digital scrambler and descrambler to thereby enable such a subscriber to communicate with all other subscribers having duplicate equipment and to do so in a secure mode.
It becomes apparent that such subscribers may also desire to directly communicate with another subscriber in a conventional manner. Hence, in order to enable a subscriber to do so, the system must have the capability of bypassing the secure communication apparatus to enable a plain text voice path to be set up between subscribers. The term plain text refers to ordinary analog speech or ordinary data prior to scrambling. While it is extremely desirable to enable such subscribers to possess the dual capacity of secure communications and plain text communications, the system must assure that the bypassing of the secure communication or scrambling equipment is achieved in a manner to prevent information which must be secure from inadvertently leaking out and hence transmitted through the plain text bypass circuit. In this manner, while the approach to bypass a scrambling circuit may appear to be a simple matter, it is imperative that the bypass circuitry does not provide a parallel path during a secure communications mode, whereby during such a mode, the actual voice data which contains highly secured information, will not be directly transmitted to the telephone line.
It is therefore an object of this invention to provide apparatus which operates to enable the transmission and reception of plain voice text and to disable this path during a secure transmission, while assuring that no secured voice data will be transmitted through the bypass apparatus. The apparatus further provides a means of monitoring the status of the bypass circuit during a secure transmission in order to verify that the path is, in fact, disabled.